A flywheel catapult for launching aircraft requires a clutch for maintaining the prime mover and flywheel disconnected from the catapult shuttle until they are completely up to speed and full power output when the clutch is suddenly engaged to drive the shuttle through its launching stroke. Various types of clutches have been proposed for such service, for example, those described in U.S. Pat. Nos. 2,672,306 and 3,305,195. Such clutches are, however, not simple, economical, dependable, rugged, and durable. An object of this invention is to provide a relatively simple, economical, dependable, rugged and durable variable torque hydraulic clutch for a flywheel catapult.